Since emission control for automobiles has been increasingly performed on a global basis, three-way catalysts have been used for removing hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO2) contained in exhaust gases. The three-way catalysts are usually composed of fine noble metal particles such as platinum (Pt), palladium (Pd), and rhodium (Rd) supported by porous oxide carriers such as alumina (Al2O3). The noble metal functions as an active point of the catalyst.
Noble metals tend to aggregate at high temperatures, such as several hundred degrees (° C.). This aggregation causes the surface area of the active point to be decreased. Hence, in order to prevent the aggregation, control of the distance between the fine noble metal particles and control of the particle diameter thereof have been considered.
As for the control of the distance between the fine noble metal particles, since aggregation thereof is liable to progress when the fine noble metal particles are supported on an exterior carrier surface, attempts have been made to also support the fine noble metal particles on an interior of the carrier surface, that is, on surfaces of pores of the carrier. Furthermore, as for the control of the particle diameter of the fine noble metal particles, since the melting point of the particles decreases with decreasing size, at least the initial size of the fine noble metal particles is required to have a certain minimum (see Ph. Buffat et al., Phys. Rev. A, Vol. 13, No. 6 (1976)). Still further, when the particle diameters of fine noble metal particles which are supported are non-uniform, the aggregation thereof is more liable to progress by using localized metal particles having a large particle diameter as nuclei (see M. Che, J. F. Dutel et al., J. Phys. Chem. 80, p 2371. (1976)).
Hence, in order to suppress the aggregation of fine noble metal particles, it is thought to be effective to use fine noble metal particles having a diameter that meets a certain minimum, and uniformly dispersing and supporting the particles on a carrier surface.
In an attempt to accomplish the above, a catalyst has been proposed in which a metal colloid is formed using a chelating agent, and in which metal particles are dispersed and supported on a carrier surface (see Japanese Unexamined Patent Application Publication No. 2000-279824, page 2). In addition, a catalyst has also been proposed in which, by using a quaternary ammonium salt as a protective colloid, a colloid salt is supported inside pores of a carrier by impregnation (see Japanese Unexamined Patent Application Publication No. 2002-1119, page 2).
However, according to the techniques described above, a polymer material is used as a protective agent for protecting the fine noble metal particles, and the polymer material has a molecular size larger than the diameter of the pores in the alumina carrier. Consequently, the fine noble metal particles cannot be deposited inside the pores of the carrier. In addition, in the case in which a quaternary ammonium salt is used as a protective colloid, a problem of stability of the colloid salt may arise. That is, for example, the colloid may be partially aggregated and then precipitated when stored for a long period of time. Furthermore, even when noble metal colloid particles are simply supported inside pores of the carrier, migration of the noble metal itself cannot be suppressed. That is, degradation of the noble metal may disadvantageously occur due to the aggregation thereof, as discussed earlier.